In U.S. Pat. No. 4,322,518, assigned to the assignee of the present invention and hereby incorporated by reference, Blizzard discloses silicone coating compositions which comprise a solventless liquid copolymeric organopolysiloxane and a diorganopolysiloxane fluid or gum containing vinyl and/or hydroxyl radicals. When cured on various substrates, these silicone compositions provide release coatings having controllable release forces for adhesives adhered thereto.
Blizzard and Swihart, in U.S. Pat. No. 4,537,829, assigned to the assignee of the present invention, teach compositions similar to those described by Blizzard, cited supra, which further comprise an organosilicon resin consisting essentially of dimethylvinylsiloxy units, trimethylsiloxy units and SiO.sub.4/2 units and a hydrosilylation catalyst. These compositions, when cured, provided improved resistance to fuels and a high degree of corrosion protection to metal substrates.
In U.S. Pat. No. 4,701,380, assigned to the assignee of the present invention and hereby incorporated by reference, Narula et al. show that compositions similar to those taught by Blizzard, based on a hydroxy-functional diorganopolysiloxane, can be cured at room temperature to provide a strongly adhering corrosion protective coating when modified with a minor amount of an adhesion promoter selected from a specific group of functional organosilanes. In order to be of commercial value, all of the above mentioned coating compositions are generally applied from an organic solvent solution and they require the further addition of a catalyst so that the time to cure the coating is not excessive.
Although the coating compositions disclosed by Narula et al., cited supra, do provide excellent corrosion protection to ferrous metals when cured thereon, these coating systems have been found to have some inherent drawbacks. Thus, this patent suggests application of the compositions to substrates by a spray method, this coating technique being well appreciated in the art for its relative speed and efficiency. And, even though airless sprayers can handle higher viscosity compositions, commercial applicators strongly prefer to have the added flexibility of using conventional siphon cup spray equipment, wherein viscosity must remain below about 1,000 cP. From a practical standpoint, this means that the above mentioned silicone coating compositions containing a diorganopolysiloxane gum must include a high proportion of organic solvents to have the desired low viscosity. Such a coating composition, further containing a cure catalyst, can be effectively sprayed by conventional equipment but this advantage is overshadowed by its high total volatile organic compounds (VOC) content. The skilled artisan will appreciate the current efforts to reduce total VOC in coating compositions of all types as there has been considerable legislation in the United States toward this end, particularly in the state of California. Furthermore, economic considerations, such as a reduction in coating composition volume and an increased film build per spray pass, provide a strong motivation to use high solids systems, the term "high solids" denoting herein a non-volatile content of 50 to about 100 weight percent or having a total VOC of .ltoreq.420 grams/liter.
An alternative way to achieve the desired low viscosity in the compositions disclosed by Narula et al. is to employ a diorganopolysiloxane having a low molecular weight (i.e., a low viscosity fluid). While a catalyzed high solids coating composition of this type can be sprayed by conventional equipment soon after it is prepared, its viscosity soon climbs out of the preferred range. This short "bath life" (i.e., the maximum time, after being catalyzed, that a batch of the coating composition can be satisfactorily sprayed) severely restricts the batch size which may be used and therefore results in inefficiencies associated with mixing extra batches of the composition and cleaning spray equipment between batches. While not wishing to be bound by any particular theory or mechanism, it is believed that the reduced bath life of such compositions is in large part due to the inherently higher functionality (reactivity) of the low molecular weight diorganopolysiloxane fluids relative to the corresponding gums. It is further believed that the functionality of the diorganopolysiloxane fluid can also react with the functional organosilane adhesion promoter in the presence of moisture. Such a reaction could explain the observed formation of a "skin" at the surface of catalyzed coating compositions prepared according to the disclosure of Narula et al. Although surface skin which is primarily due to solvent evaporation can generally be re-dissolved by mixing it back into the coating composition, it has been found that this is not always possible in the case of sprayable high solids coating systems of Narula et al. When this skin can not be re-dispersed, it tends to clog filters and nozzles in the spray equipment and can result in poor surface quality of the coating. Again, not wishing to be bound by a particular theory, it is believed that the apparently gelled skin results from the interaction of the hydroxy-functional diorganopolysiloxane with the functional organosilane adhesion promoter in the presence of surface moisture.